Integrated circuits (ICs) have numerous circuit components interconnected to provide the desired functions. One type of circuit component is a voltage dependent capacitor, referred to as a varactor. Varactors are commonly used in radio frequency (RF) ICs, for example, forming voltage controlled oscillators (VCOs) or filters. FIG. 1 shows a conventional varactor 100 having a structure of a capacitor 110. The capacitor includes a first electrode 112 (cathode) and a second electrode 118 (anode). A first terminal 160 is coupled to the cathode and a second terminal 170 is coupled to the anode. The anode and cathode are separated by a dielectric region 114. The width of the dielectric region determines the capacitance of the varactor. In general, the wider the dielectric region, the lower the value of the capacitance. Additionally, the larger the electrode area, the higher the capacitance value. The anode and cathode of the varactor can be formed at a p-on-n or n-on-p junction, with the depletion region at the junction serving as a dielectric region.
FIGS. 2a-b show cross-section views of a conventional n-on-p (NP) junction varactor 100. The junction varactor is formed on a substrate 205. The substrate includes a p-well 218. Shallow trench isolations (STIs) 280 define first and second regions 162 and 172a-b. A heavily doped n-type region is provided in the first region, creating an NP junction 214 with the p-well. The n-doped region serves as the anode 118 while the p-well serves as the cathode 112. Contact to the p-well is achieved through heavily doped p-type regions at the surface of the second region. The input and bias voltages are applied to the cathode and anode terminals respectively.
A depletion region, indicated by dotted lines 216a-b, occurs at the NP junction. The capacitance of the varactor corresponds to the width of the depletion region, which can be varied by adjusting the voltages at the terminals. When the NP junction is at zero bias (i.e., Vin=Vbias), the depletion width is at its minimum, corresponding to a maximum capacitance (Cmax), as indicated in FIG. 2a. The width of the depletion region widens as the reverse bias across the junction is increased. Widening the depletion region decreases the capacitance of the varactor. When the maximum reverse voltage is applied, the depletion width is at its widest which corresponds to a minimum capacitance (Cmin), as shown in FIG. 2b. 
An important factor is the tuning range of the varactor, which corresponds to the capacitance range in which the varactor operates and is defined by the ratio Cmax/Cmin. Generally, it is desirable for a varactor to have a large tuning range to provide better functionality. Furthermore, particularly with mobile products, a high quality factor is desirable since this corresponds to low power consumption. Although junction varactors have better linearity compared to MOS-type varactors, they suffer from a smaller tuning range.
From the foregoing discussion, it is desirable to provide a junction varactor with a large tuning range and high quality factor.